EcoMotors International, a startup based in Troy, Michigan, has a new
approach to an old idea--the two-stroke engine--which it says is up to
50 percent more efficient than most vehicle engines and pollutes far
less than a conventional two-stroke engines.

The company recently received a combined $23.5 million in investment
from Bill Gates and Khosla Ventures. That money will go toward
development of EcoMotors's opposed piston, opposed cylinder (OPOC)
engine. The engine uses two piston movements per cycle, instead of four,
and each cylinder contains two opposing pistons, instead of one. A
single crankshaft sits in between pairs of cylinders. The design relies
on precise computerized control of all the components.

A conventional car engine takes four piston movements, or strokes, to
go through intake, compression, combustion, and exhaust. In a
two-stroke cycle, these stages are completed with just two piston
movements, delivering twice as many power strokes per revolution and
requiring fewer parts. But two-stroke engines tend to spew out more unburned fuel in the exhaust, which is why the four-stroke design became more common.

Putting two pistons inside each cylinder also means that each piston
only travels half as far as it normally would in a two-stroke engine,
allowing the engine to run faster. Having half as many parts as a
conventional engine (the OPOC does not have cylinder head or valve-train
components, and it has fewer bearings) helps to reduce friction and
heat loss. These factors, combined with "a long list of 1 and 2 percent
improvements" in other areas, says Ecomotors's CEO Don Runkle, account
for a 15 percent efficiency improvement.